A linear regulator operates to provide a regulated output voltage from an input voltage. A linear regulator includes a pass transistor to provide the regulated output voltage from the input voltage, where the base or gate of the pass transistor is adjusted through negative feedback to regulate the output voltage.
A typical linear regulator requires a fair amount of headroom to operate. However, a low-dropout regulator (LDO) is designed to operate with a low dropout compared to a typical linear regulator (where dropout refers to the voltage difference between the input voltage at the regulated output voltage). In a low-dropout regulator, the pass transistor is typically a p-type enhancement mode transistor.